It appears that just like with stock Android 6.0 on the new Nexus 5X and 6P, those old devices are getting the quick launch option to double-press the power button to jump straight into the camera at any time. In theory, this will make it faster to get to the camera, no matter what you're doing on your device, especially one-handed. Unfortunately, in practice, the gesture can be a bit finicky. Because it is tied to the power button, we've found on the Nexus 6P that the camera may launch, but then the device will go to sleep, or the device will go to sleep without launching the camera at all. And, if you have extra security on your lockscreen, there can be weirdness there where it asks you to unlock when you should just go straight into the camera.
Those missteps are admittedly not the norm, but Google does still have a few kinks to work out, and the gesture doesn't work quite as well as the Motorola double-twist gesture. Still, a nice update for a bunch of devices!
source: Android Police